Water-absorbing resins are widely used in sanitary and hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, crosslinked and partially neutralized copolymers of isobutylene and maleic anhydride, saponification products of vinyl acetate-acrylic acid copolymer, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
These polymers, and others, are known in the art by various names, such as superabsorbent polymers, hydrogels, hydrocolloids, and water-absorbent hydrophilic polymers, for example. As used herein, the term “SAP” refers to a superabsorbent polymer, and collectively refers to such water-absorbing materials. As used herein, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, more specifically, particles containing from no water up to an amount of water less than the weight of the particles, and typically less than about 5%, by weight, water. The terms “SAP gel,” “SAP hydrogel,” or “hydrogel” refer to a superabsorbent polymer in the hydrated state, more specifically, particles that have absorbed at least their weight in water, and typically several times their weight in water.
SAPs are lightly crosslinked hydrophilic polymers, and are discussed generally in Goldman et al. U.S. Pat. Nos. 5,669,894 and 5,59,335, each incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.
An SAP typically is neutralized at least about 25 mole percent, preferably at least about 50 mole percent, and usually about 70 to 80 mole percent, to achieve optimum absorbency. Neutralization can be achieved by neutralizing the acrylic acid monomer before polymerization of the monomer, or the polymer can be neutralized after the polymerization reaction is substantially complete. After polymerization and internal crosslinking of the monomer, followed by partial neutralization, e.g., 50-100 mole percent neutralization, preferably 70 to 80 mole percent neutralization, the polymer is subdivided, e.g., shredded or chopped, for more efficient drying, then dried and milled to a desired particle size. The polymer preferably then is surface crosslinked and again dried to form the final product.
Many improvements have been made in the performance and properties of SAPs, such as in gel strength and reabsorbing capacity. However, investigators are continually searching for additional improvements, for example, in SAP permeability and fluid acquisition rates, such that the amount of cellulosic fiber in a diaper core can be reduced. Reducing the amount of fiber permits the manufacture of a thinner diaper, which is extremely important in adult incontinence articles.
Clays and other mineral products have been added to SAPs in an attempt to improve SAP performance. For example, the addition of finely divided amorphous silica, such as AEROSIL®, available from Degussa, DE, or CAB-O-SIL®, available from Cabot Corporation, or a bentonite onto the surface of SAP powders or granules is known. U.S. Pat. Nos. 5,140,076 and 4,734,478 disclose the addtion of silica during surface crosslinking of dry SAP powders. U.S. Pat. No. 4,286,082 discloses mixtures of silica and an SAP for use in hygiene articles.
JP 65 133 028A and JP 61 017 542B disclose mixtures of hydrophobic silicas and absorbent polymers. EP 0 341 951, U.S. Pat. No. 4,990,338, and U.S. Pat. No. 5,035,892 disclose the use of silica in the production of antimicrobial absorbent polymers. U.S. Pat. No. 4,535,098 and EP 0 227 666 disclose the addition of silica-based colloidal substances to enhance the gel strength of SAPs.
Generally, in mixtures of dry SAP particles with a silica powder, the silica adheres to the SAP particle surfaces and alters the surface properties of the SAP particles, but not their intrinsic absorption properties. For example, the silica powder is hydrophilic or hydrophobic, which primarily influences the rate at which a fluid is absorbed by the SAP particles.
WO 99/64515 discloses the preparation of SAPs by polymerizing olefinically unsaturated carboxylic acids and adding a silicate before, during, and after polymerization. The swollen polymer particles have improved mechanical stability and enhanced permeability. However, because the silicate framework lacks a charge, no osmotic pressure can be generated. This neutral silicate framework does not contribute to the osmotic swell pressure of the hydrogel, and fluid absorbency is adversely effected.
WO 99/55767 discloses ionically crosslinked SAPs obtained by polymerizing carboxyl-containing monomers and adding aluminate ions before, during, and after polymerization. The presence of ionic crosslinked sites provides improved gel stability under mechanical load. However, the salt stability of these hydrogels is inadequate, and a premature collapse of the network structure occurs at a high salt content.
Other patents and applications disclosing SAP particles and a clay include GB 2,082,614 disclosing a dry, solid, water-swellable absorbent composition prepared by blending dry SAP particles and 1% to 75%, by weight of the blend, of an extender material selected from uncrosslinked cellulose derivatives, starch, certain clays and minerals, and mixtures thereof.
U.S. Pat. No. 4,500,670 discloses water-absorbent compositions containing a water-swellable SAP and an inorganic powder, preferably a clay. The compositions are prepared by physically blending the inorganic powder and SAP particles after the SAP is polymerized and crosslinked.
U.S. Pat. No. 4,735,987 discloses polymerization of a partially neutralized acrylic acid via an inverse suspension polymerization process, with the simultaneous addition of a crosslinker and an inorganic material, e.g., a clay, to the polymer bead suspension, followed by azeotropic dehydration. Crosslinking occurs in the presence of the clay during the dehydration step. The resulting product has a high volume expansion after swelling in saline solution.
U.S. Pat. No. 4,914,066 discloses pellets containing 0.5 to 15 wt. % SAP and 85 to 99.5 wt. % bentonite clay, prepared by mixing the SAP and bentonite in the presence of water, then compressing and extruding the blend through an orifice to form the pellets, followed by drying.
WO 91/12029 and WO 91/12031 disclose compositions containing an SAP combined with odor-controlling agents, preferred zeolites, by means of a binder. The SAP particles are coated with the zeolite in presence of a binder in a fluidized bed coating apparatus, or are admixed with dry SAP particles and water, and the mixture is dried by heating.
U.S. Pat. No. 5,419,956 discloses mixtures of SAP fines with an inorganic powder, like silica or clay.
U.S. Pat. No. 5,733,576 discloses a process of producing absorbing agents containing (a) a water-swellable, synthetic polymer or copolymer, and (b) a natural or synthetic polymeric compound which at normal temperature is a pourable powder and is partially soluble or insoluble in water. The absorbing agents can contain clay as a neutral filling agent.
EP 0 799 861 discloses a particulate deodorant composition containing an SAP and powdery zeolite dispersed within the SAP resin particles. The composition is prepared by kneading a water-absorbent resin and a zeolite powder in the presence of water, followed by drying and grinding.
U.S. Pat. No. 6,124,391 discloses SAP particles containing 0.2 to 10 wt % of an inorganic powder, e.g., a clay, having improved anticaking properties, wherein more than 60 wt % of the particles are larger than 300 μm. Clay is added before, during, or after a surface crosslinking step.
WO 00/72958 discloses a process for producing a networked polymer/clay alloy for use in a personal care article. The process comprises the steps of:                (a) preparing a monomer/clay mixture by mixing at least a monomer, clay particles, a crosslinking agent, and a mixing fluid in a vessel;        (b) exposing the monomer/clay mixture to a polymerization initiator; and        (c) polymerizing the monomer/clay mixture to form a networked polymer/clay alloy.        
WO 01/13965 discloses water-absorbent polymers containing silicium-rich zeolites for odor control. The silicium-rich zeolites can be added to the monomer solution, to the SAP gel, or in the surface crbsslinking step.
WO 01/32117 discloses an SAP composition containing a partially neutralized SAP wherein at least 30% of the functional groups of the polymer are in free acid form, and a layered double hydroxide anionic clay, e.g., hydrotalcite clays. In the examples, the composites were prepared by powder/powder mixing.
WO 01/68156 discloses a hydrophilic swellable hydrogel-forming polymer containing alumosilicate and having enhanced permeability and improved odor-control properties. The alumosilicates can be added before, during, or after polymerization.
However, a need still exists for SAP-clay particles having improved fluid permeability when in the swollen state. The above-described compositions containing an SAP and an inorganic material, like a clay, have not met this need. Accordingly, the present invention is directed to improving the permeability and absorption rates of SAP particles by introducing a clay into the SAP hydrogel. It has been found that the addition of a clay to an SAP hydrogel prior to neutralizing the hydrogel facilitates drying of the hydrogel, and can significantly improve SAP performance with respect to rate of fluid absorption by a diaper core containing the SAP-clay particles, and fluid permeability through swollen SAP-clay particles.
Therefore, the present invention is directed to improving SAP absorption rate and permeability performance, without adversely affecting other fluid absorption and retention properties of the SAP particles, by the addition of a clay during the manufacturing process.